Many polymeric viscosity index improvers are available for lubricating oils but most of these viscosity index improvers do not have sufficiently high shear stabilities to be acceptable in gear oil service. Commercial gear oils viscosity index improvers include polyisobutylenes and polymethacrylates. To be acceptable gear oil viscosity index improvers, both of these types of polymers must be presheared to a uniform low molecular weight. This preshearing adds expense to the manufacturing process. Further, these presheared polymers are not efficient as thickeners, and a relatively large amount of either is required to impart an acceptable viscosity index improvement to a base gear oil.
Another prior art gear oil viscosity index improver is disclosed in U.S. Pat. No. 4,082,680. This patent describes a relatively low molecular weight hydrogenated butadiene-styrene diblock copolymer. The polymer is 30 to 44 weight percent butadiene and has a molecular weight within the range of 12,000 to 20,000. This is a lower molecular weight version of a diblock copolymer which is known to be useful as a viscosity index improver for motor oils. Like the presheared viscosity index improvers, the low molecular weight results in a relatively low thickening efficiency. A high concentration is therefore required to impart an acceptable viscosity index for multigrade gear oils.
Hydrogenated conjugated diolefin polymers having a star, or radial configuration are known to be useful as viscosity index improvers for motor oils, but, again, these motor oil viscosity index improvers are not acceptable as gear oil viscosity index improvers due to low shear stability. Such motor oil viscosity index improvers are disclosed in U.S. Pat. No. 4,156,673. The star polymers are generally oil soluble to much higher molecular weights than linear counterparts. Because higher molecular weight polymers are more efficient thickeners this results in less polymer being required. This results in a significant cost advantage for the use of hydrogenated radial conjugated diolefin polymers as motor oil lubricating oil viscosity index improvers. The higher molecular weight star polymer is also disclosed as being more shear stable than linear counterparts, but shear stabilities sufficient for gear oil service are not disclosed.
It is therefore an object of the present invention to provide a gear oil composition which has excellent shear stability, an acceptable viscosity over a wide temperature range and which requires a lower level of polymer additive than the gear oil compositions which comprise prior art polymeric viscosity index improvers. In another aspect it is an object of this invention to provide a method to improve the viscosity index of a gear oil and also maintain an acceptable shear stability.